1. Field of the Invention
The present invention relates to a high frequency power amplifier for dual band systems used, for example, in mobile telephones, and relates more particularly to a high frequency power amplifier of heterojunction bipolar transistors (HBT) having a bias switching circuit for switching a bias circuit to select a power amplifier.
2. Description of the Prior Art
Monolithic microwave integrated circuits (MMICs) and modules (hybrid ICs and multichip modules) using GaAs metal-semiconductor field effect transistors (MESFETs), GaAs high electron mobility transistors (HEMTs), or GaAs-based HBTs are widely used in power amplifiers for mobile communications devices. Compared with a conventional FET, a GaAs-based HBT does not require a negative gate bias voltage, enabling a single power supply operation, and can effect an on/off operation similarly to a Si-MOSFET but without an analog switch on the drain side. A further advantage is that GaAs-based HBTs can be used to achieve a smaller power amplifier generating a rated output power with a higher output power density than is possible with a conventional FET power amplifier. GaAs-based HBTs are therefore considered promising as power elements in future mobile communication devices.
With recent sharp increases in demand for mobile phones, telephones and services enabling access to two systems using a single handset have been introduced. In Europe, for example, there are two mobile telephone services: GSM 900 (Global System for Mobile Communications), a 900 MHz system that is the most widely used mobile telephone service in Europe, and DCS 1800, a competing system that operates in the 1800 MHz band. Japan also has two systems: Personal Digital Cellular (PDC), a 900 MHz mobile telephone system, and PHS (Personal Handyphone System), a 1900 MHz digital cordless telephone system. So-called dual band mobile phones that can switch between two different telephone services are already available.
FIG. 10 is an exemplary circuit block diagram of a high frequency power amplifier for use in a conventional dual band mobile phone.
Referring to FIG. 10, GSM 900 transistors TrA1 to TrA3, that is, HBTs for amplification in the 900 MHz band, are biased by a first biasing circuit 201. GSM 1800 TrB1 to TrB3, that is, HBTs for amplification in the 1800 MHz band, are biased by a second biasing circuit 202.
Bias switching circuit 203 exclusively selects and operates either the first biasing circuit 201 or second biasing circuit 202 based on the signal level of a two-valued input signal Vmod applied to the Vmod terminal. The bias switching circuit 203 also passes an output control signal based on the signal level of the input signal Vpc applied to the Vpc terminal to the first biasing circuit 201 or second biasing circuit 202 to adjust the level of the output signal passed from output terminals OUT1 and OUT2. When input signal Vpc is 0 V, the bias switching circuit 203 stops the operation of both the GSM 900 power amplifier and the GSM 1800 power amplifier regardless of input signal Vmod.
FIG. 11 is a typical circuit diagram of the bias switching circuit 203 used in the conventional high frequency power amplifier shown in FIG. 10. As shown in FIG. 11, this bias switching circuit 203 comprises a logic circuit 210 of CMOS or other silicon circuit elements, and an output buffer 211. The output buffer 211 comprises a plurality of npn transistors TrC1 to TrC4 made in silicon. The logic circuit 210 converts the input signal Vpc supplied from a DA converter (not shown in the figure) according to the signal level of input signal Vmod, and outputs either signal V900 or V1800 through output buffer 211. The bias switching circuit 203 thus outputs signal V900 or V1800 to select a particular power amplifier and controls the signal level of the output signal from the selected power amplifier based on input signal Vpc, and stops operation of the other (unselected) power amplifier regardless of input signal Vpc.
It is to be noted that while the first biasing circuit 201 and second biasing circuit 202 shown in FIG. 10 are made with silicon elements, those shown in FIG. 12A and FIG. 12B are made using HBT as known from the literature. The biasing circuit shown in FIG. 12A can compensate for a change in the base-emitter voltage Vbe according to the temperature coefficient of the power amplification HBT in FIG. 10 by means of transistor TrD2, as can the biasing circuit shown in FIG. 12B using transistors TrD2 and TrD3.
It is to be noted that all transistors TrD1 to TrD3 in FIGS. 12A and 12B are HBTS, and either signal V900 or V1800 in FIG. 11 can be applied to either biasing circuit through the biasing circuit input terminal Vin.
Unlike an FET, however, an HBT operates by amplifying the base current, and requires a base current supply between several 10 to 100 mA to achieve the high 2 W to 4 W output of a GSM phone. It is, however, also difficult to directly obtain such a high base current from a standard Si-CMOS IC because the output current used to assure a specific output voltage from a standard CMOS device is a maximum of several milliamperes.
Furthermore, two power amplifiers can be easily controlled using input signals Vpc and Vmod due to the general utility of the logic circuit design when silicon elements are used in the bias switching circuit, but a separate control circuit using silicon elements is required for controlling devices other than the HBT power amplifier.
When silicon elements are used in the bias switching circuit and biasing circuits, or just the bias switching circuit, these circuits cannot be integrated to an MMIC comprising HBTs. A separate silicon chip is therefore needed, and this reduces yield during mass production.